Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Merging galaxy clusters: radio and X-ray studies • hierarchical structure formation in the universe • still ongoing at z = 0 • X-ray substructure • radio emission • cluster weather • cosmological shocks • weather in cluster gas IMPRS, April 8 themes of a commencing graduate school ... Clusters of galaxies • groups and clusters of galaxies = largest gravitationally bound and collapsed systems in the universe • groups: 3 ··· 30 galaxies; • clusters: up to a few 1000 • R ~ 2 Mpc • M ~ 1014 ··· 1015 M • Local Group: ~ 35 members • MW, M31, M33; all others dwarf galaxies • Virgo Cluster: ~ 2100 members (Binggeli et al. 1985) • Abell Catalogue (POSS + ESO SSS): m1 m2 1682 clusters (Abell 1958) 4073 “ (Abell, Corwin & Olowin 1989) • criterion: 50 galaxies with m3 m m3 + 2 • contained within ‘Abell Radius’ A = 1.5’/z, i.e. RA = 1.5 h-1 Mpc • covers 0.028 < z < 0.20 cD B F L C I - single dominant cD galaxy (A2029, A2199) - dominant binary, like Coma - flattened (IRAS 09104+4109) - linear array of galaxies (Perseus) - single core of galaxies - irregular distribution (Hercules) m3 m3+2 50 Structures of galaxy clusters Böhringer (1996) • 3 mass components: visible galaxies, ICM, DM - galaxies : ~ 3% (optical, IR) - ICM : 10 ··· 15 % (X-rays) - DM : ~ 80% (v , grav. lensing) Coma belief until ~ mid 80’s: “clusters are simple ...” however: ample evidence for substructure, rendered visible most convincingly in X-ray regime ‘true-nature-images’ of clusters! • radial variations of centroids • twists in X-ray isophotes (e.g. Coma Cluster!) • non-Gaussian skewed or even bimodal f(v)’s A3528 A578 A1569 X-ray morphologies of clusters Optical techniques barely disclose gravitational potential in nearby clusters unless these are rich (too few test particles); distant ones: lensing ... M G 3 r2 r 1 1 X-rays: continuous mapping of in galaxy clusters • systematic imaging : EINSTEIN, ROSAT • hígh spatial rersolution : CHANDRA • “ spectral “ : XMM • mapping of T : ASCA M ( r ) G mH k T r 2 dr dr d (log ) d (log T ) Fornax Cluster Abell 2256 • systematic X-ray survey of galaxy clusters: REFLEX (Böhringer et al. 1999) • basically 1000s of clusters, mostly with but few ( 100) photons... • 452 clusters, 53% Abell (only!) • for m = 0.3 cluster mass contributes ~ 6% to total matter in the Universe Radio emission from clusters of galaxies Another diagnostic tool of cluster physics: radio emission: synchrotron radiation Clarke et al. (1999) • is the IGM/ICM magnetizied? • how (and when) did it get magnetized? AGN (‘standard’) dwarf galaxies (Kronberg et al. 1999) • evidence for B-fields: - radio halos & relics (e.g. Feretti 1999) Isyn N0 B 1 - Faraday rotation 5 G (Clarke et al. 1999) RM 2 , RM 0.81 Enßlin & Biermann (1998) ( cm3 ) ( G ) ( kpc) ne B ds 1 G (e.g Enßlin & Biermann 1998; Tsay et al. 2002) IC results not yet conclusive - Inverse Compton emission I I IC syn B (1 ) ( kT ) 3 E IC B (1 ) Isyn IIC Dixit deus: “Fiat lux (campus magnetibusque)” Thierbach et al. (2002) Feretti & Giovannini (1998) • radio ‘halos’ : central, diffuse, polarization < 5% • radio ‘relics’ : peripheral, 20% polarized • no obvious particle/energy sources • steep(ening) spectra at higher frequencies • how frequemt? many if searched for with scrutiny! Röttgering et al. (1999) ‘Weather stations’ in galaxy clusters ~ 10% of galaxies in clusters produce significant radio synchrotron emission (Pν 1023 W Hz-1 at 20 cm) • jets of radio plasma ejected from galaxy cores, forming lobes and tails probe relative gas motions over 100’s of kpcs (NATs, WATs) • former belief: tails simply trace ballistic motions of galaxies when radio plasma is exposed to ICM ram pressure (radius of curvature R , jet radius rj , jet velocity vj , galaxy velocity vg , density of jet j , density of ICM ICM density of ICM): R rj j V j ICM Vg 2 2 • however: ~90% of WATs & NATs in clusters with X-ray substructure; correlation between elongations in X-rays and bending of radio tails • cluster mergers bulk flow ram pressure bends of radio tails and distortion of X-ray surface brightness • Perseus Cluster (Sijbring 1994): low-frequency kinks and bends suggest highly non-ballistic motions caused by turbulent motions of the ICM plasma! ’high winds’ • synchrotron ages from break frequency b (GHz), equipartition magnetic field Beq (G), equivalent magnetic field of CMB BCMB (G): 108 b1/ 2 Beq BCMB 2 2 Beq yr Perseus at 610 MHz 3C465 Radio sources are - barometers to measure ICM pressure - anenometers to measure cluster winds (the only measure so far!) Radio relics: revived particle pools • classical cases of peculiar peripheral & extended radio sources: - A2256 (Röttgering et al. 1994; Röttgering et al. 1994) - 1253+275 in Coma (Giovannini et al. 1991) • common properties: - peripheral - steep spectrum - linearly polarized ordered B-field A 2256 1465 MHz • degree p of polarization depends on compression ratio of shock, on particle spectrum, N(E) · dE ~ E-s · dE, and on the orientation of shock w.r.t. observer: p s7 3 f ( R, ) s 1 • origin of relic: several radio galaxies in the vicinity of 1253+275 (Giovannini et al. 1985); loss << kin solved by large-scale accretion shocks (Enßlin et al. 1998); low galaxy density turbulent reacceleration by galactic wakes ruled out. • 16 clusters with known relics (compilation in Slee et al. 2001) • only 4 clusters with relics have measured polarization (see Enßlin et al. 1998). A 2256 Xray. A 2256 opt. Coma Cluster 327 MHz Cosmological shock waves at intersecting filaments of galaxies • NGC315: a giant (~ 1.3 Mpc) radio galaxy (GRG) with odd radio lobe (Mack 1996; Mack et al. 1998). - morphology: precessing jets (Bridle et al. 1976), but western one with peculiar bend towards the host galaxy - unusually flat radio spectrum in western lobe: first steepens (as expected), then flattens to high 0.7 (S ~ --). - strong linear polarization: p 30%. • Enßlin et al. (2000): originally symmetric radio galaxy “falling” into an intergalactic shock wave, along with its environment. • compression reacceleration of particles strong alignment of magnetic field & increased synchrotron emissivity • origin of large-scale gas flow and shock wave? • NGC315 located within Pisces-Perseus Supercluster • Enßlin et al. (2000) identify filaments of galaxies with rather different velocity dispersions (redshifts from CfA survey, Huchra et al. 1990, 1992, 1995): - filament I : v 400 km s-1 - filaments II - V : v 90 ···· 220 km s-1 if gas has comparable v , this translates into k ·TI 280 eV k ·TII-V 15 ···· 85 eV • from theory of shocks (Landau & Lifschitz 1966) temperature jump T1 /T2 3.3 ···· 20 compression ratio R 2.9 ···· 3.8 pressure jump P1 /P2 9.6 ···· 75 • O’Drury (1983): 0.54 ···· 0.79 expected N(E) · dE ~ E-s · dE s S ~ - R2 R 1 • gas in one of smaller filaments (II - IV) may get heated by shock wave when flowing into deeper gravitational potential of main filament (I). • cosmological shockwave in NGC315 is putative; onfirmation requires - deep X-ray imaging to see heated gas - low-frequency search for relic-type, diffuse radio emission over entire shock region NGC315 I II III IV V view from ‘above’ ‘Weather forecast’ • head-tail (or other extended) radio sources must be studied, along with environment (X-ray studies) • search for radio relics in cluster merger candidates at low frequencies, with scrutiny of spectral aging and linear polarization: essentially all cluster merger candidates should exhibit this.... • new-generation X-ray telescopes with high spatial & spectral resolution studies of gas motions • to be compared with high-fidelity numerical simulations that take advantage from - new-generation supercomputers - adaptive mesh refinement - higher mass resolution - MHD Röttiger et al. 1998 Landau & Lifschitz (1966): pressure and temperature ratios between down- and upstream region (inside and outside cluster shock front) are: P2 4 R 1 P1 4R T2 P1 1 T1 P2 R • GRGs: probes of tenuous IGM • Clarke et al. Method (RM in clusters) • Laing-Garrington • ram pressure stripping (Virgo) • • how much mass in form of hot gas? • importance of ghosts? • • primary/secondary/in situ